In the fabrication of semiconductor wafers several process steps require contacting the wafers with fluids. Examples of such process steps include etching, photoresist stripping, and prediffusion cleaning. Often the chemicals utilized in these steps are quite hazardous in that they may comprise strong acids, alkalis, or volatile solvents.
The equipment conventionally used for contacting semiconductor wafers with process fluid consists of a series of tanks or sinks into which cassette loads of semiconductor wafers are dipped. Such conventional wet processing equipment poses several difficulties.
First, moving the wafers from tank to tank may result in contamination, which contamination is extremely detrimental to the microscopic circuits which the fabrication process creates. Second, the hazardous chemicals and deionized water which are used have to be regularly replaced with new solutions, usually introduced to the tank by bottle pour, chemical distribution or from building facilities in the case of deionized water. The chemicals generally are manufactured by chemical companies and shipped to the semiconductor manufacturing plant. The chemical purity is thus limited by the quality of the water used by the chemical manufacturers, by the container used for shipping and storing the chemical and by the handling of the chemical.
Moreover, as chemicals age, they can become contaminated with impurities from the air and from the wafers. The treatment of the last batch of wafers prior to fluid rejuvenation may not be as effective as treatment of the first batch of wafers in a new solution. Non-uniform treatment is a major concern in semiconductor manufacturing.
Some of the fluid contact steps of semiconductor manufacture include removal of organic materials and impurities from the wafer surface. For example, in the manufacture of integrated circuits, it is customary to bake a photoresist coating onto a silicon wafer as part of the manufacturing process. This coating of photoresist or organic material must be removed after processing.
Generally, a wet photoresist strip process is performed by a solution of sulfuric acid spiked with an oxidizer of either hydrogen peroxide or ozone. This process is referred to in U.S. Pat. Nos. 4,899,767 and 4,917,123, issued to CFM Technologies. However, there are many disadvantages to using a solution of sulfuric acid and an oxidizer to strip photoresist from wafers during semiconductor manufacture. First, the by-product of the resist strip reaction when hydrogen peroxide is used as the oxidizer is water, which dilutes the concentration of the bath and thereby reduces its ability to strip photoresist. Second, this process operates at a high temperature, generally between 80.degree. and 150.degree. C., typically above about 130.degree. C., which mandates the use of special heat resistant materials and components in order to house, circulate and filter the solution, as well as requires extra energy to conduct the cleaning process. Third, the solution is hazardous to handle and dispose of and expensive to manufacture, transport and store.
Moreover, due to the build-up of impurities both dissolved and undissolved in the process bath, the solution must be changed periodically. Typically, the interval for chemical change out is about every eight hours. Because the chemical adversely affects the drain plumbing, the solution must be cooled to less than about 90.degree. C. prior to disposal. Thus, use of this photoresist stripping process requires either the use of additional tanks to contain the hot solution or the shut down of the process station during the chemical change period, reducing wafer throughput and increasing cost of ownership.
Finally, after use of a sulfuric acid solution for removal of photoresist, the wafers must be rinsed in hot deionized water since sulfate residues may crystallize on the wafer during processing causing process defects.
Another process often utilized for the removal of organic and metallic surface contaminants is the "RCA clean" process which uses a first solution of ammonium hydroxide, hydrogen peroxide, and water and a second solution of hydrochloric acid, hydrogen peroxide, and water. The RCA cleaning solutions typically are mixed in separate tanks. The wafers are first subjected to cleaning by the ammonium hydroxide solution, then are moved to a rinse tank, then to a tank containing the hydrochloric acid cleaning solution, and then to a final rinse tank. This process, like the sulfuric acid process, has the disadvantage of using strong chemicals. Moreover, the wafers are exposed to air during the transfers from tank to tank, allowing for contamination. Finally, the use of peroxide may cause the wafers to suffer aluminum contamination from the deposition of aluminum in the high pH ammonium hydroxide solution which is not totally removed in the hydrochloric solution.
Various approaches have been taken to improving the processes and equipment used to treat semiconductor wafers with fluid. These attempts to improve on present processes generally involve either a change in equipment or a change in the process chemicals.
One approach to eliminating problems with contamination of wafers during fluid treatment is disclosed in U.S. Pat. Nos. 4,778,532, 4,795,497, 4,899,767 and 4,917,123. These patents describe an enclosed full-flow method and apparatus which allows the process fluids to flow sequentially and continuously past the wafers without movement or operator handling of the wafers between processing steps. However, these patents still teach the use of hazardous chemicals to perform the fluid treatment and cleaning of the wafers. Moreover, the equipment needed for the enclosed apparatus is limited in wafer throughput since all process sequences are performed in the same vessel with concentrated solutions.
U.S. Pat. No. 4,899,767 teaches the use of a separate mixing tank for preparing the sulfuric acid and oxidizer solution, which solution must then be delivered into the treatment tank. The reason for the separate tank is to eliminate the possibility of an explosion due to pressure build up from the decomposition of hydrogen peroxide into oxygen and water.
U.S. Pat. No. 5,082,518, issued to SubMicron Systems, Inc., describes a different approach to improving the sulfuric acid and oxidizer process of cleaning semiconductor wafers. The system in this patent provides a gas distribution system which includes a sparger plate with diffusion holes for distributing gas throughout the bath in the tank. Thus, rather than using a separate tank for mixing as in U.S. Pat. No. 4,899,767, the SubMicron patent provides an apparatus which distributes ozone directly into the treatment tank containing sulfuric acid. It has been found, however, that this diffusion system suffers several disadvantages. First, the efficiency of ozone distribution and absorption into the water is lessened by the large bubbles of ozone produced by the apparatus. The amount of ozone absorbed is important to its ability to react with the sulfuric acid to remove organic materials from the wafers. Moreover, the type of diffusing element described in U.S. Pat. No. 5,082,518 is believed to not uniformly distribute ozone throughout the tank. Finally, as with previous attempts to improve cleaning processes for wafers, hazardous chemicals are required, creating handling and disposal problems.
An approach to eliminating the problem of the use of hazardous chemicals is set forth in Ohmi et al, J. Electrochem. Soc., Vol. 140, No. 3, March 1993, pp. 804-810, which describes the use of ozone-injected ultrapure water to clean organic impurities from silicon wafers at room temperature. However, this process also suffers several disadvantages. Ohmi et al provides only a process for the removal of a very thin layer of organic material, i.e., a layer of surfactant left from the lithography process. The process described by Ohmi et al could not remove photoresist in a reasonable time frame. The process was intended for, and works on organic contamination layers of less than 50 Angstroms. It is too slow to work on organic contamination layers of 50-250 mils. Thus, a process which can quickly and effectively remove organic materials of all thicknesses from semiconductor wafers without the use of hazardous chemicals is still not available in the art.
A process for the removal of organic materials during semiconductor manufacture which can avoid the foregoing problems while providing effective removal of organic materials would be of great value to the semiconductor industry. Further, an apparatus for conducting such a process which eliminates the need for multiple tanks would also be of great value to the industry.
After the cleaning process described above is completed, the wafers are usually rinsed and dried. Conventionally, semiconductor wafers are dried through centrifugal force in a spin rinser-drier. Because these devices rely on centrifugal force, their use results in several problems. First, there are mechanical stresses placed on the wafers which may result in wafer breakage. Second, because there are moving parts inside a spin-rinser-drier contamination control becomes a difficult problem. Third, since the wafers conventionally travel at high velocity through dry nitrogen, static electric charges develop on the wafer surfaces. Since oppositely charged airborne particles are quickly drawn to the wafer surfaces when the spin-rinser-drier is opened, particle contamination results. Fourth, it is difficult to avoid evaporation of water from the surfaces of wafers during the spin process. Because even short periods of contact of ultrahigh purity water with wafer surfaces will permit the water to dissolve minute quantities of silicon or silicon dioxide, evaporation of water containing dissolved silicon (deposited silicon or silicon dioxide) or silicon dioxide will result in a streak or spotting on the wafer surface. Streaking or spotting often ultimately results in device failure.
Another method for drying semiconductors is vapor drying with a vapor such as isopropyl alcohol ("IPA") such as is disclosed in U.S. Pat. No. 4,984,597. In this process, a wafer previously rinsed with sequential baths of deionized water is thereafter exposed to a superheated vapor of isopropyl alcohol (IPA) in a fully enclosed vessel. The drying vapor directly displaces the water from the surfaces at such a rate that substantially no liquid droplets are left on the surfaces after replacement of the water with drying vapor. The drying vapor is then purged with a stream of dry, inert gas. The drying vapor is miscible with water, forms a minimum boiling azeotrope with water and works best when the wafer is at the same temperature as the wafer. The drying process takes several minutes.
U.S. Pat. No. 4,778,532 discloses a two step chemical drying process. First, the rinsing fluid, preferably water is driven off the wafers and replaced by a nonaqueous miscible drying fluid. Second, the nonaqueous drying fluid is evaporated using a predried gas, preferably an inert gas such as nitrogen at a low flow velocity. In the process disclosed, a nonaqueous drying fluid is heated to form a vapor which enters the wafer tank and condenses on the interior surfaces, including the surface of the wafer to be dried. The condensed vapor displaces liquid rinsing fluid from the surface of the wafers. It is disclosed that the transfer of drying fluid from a drying fluid liquid source to the interior of the wafer tank as a gas is preferred.
European Patent 0 385 536 A1 discloses a drying method wherein a substrate is immersed for some time in a bath containing a liquid and is then taken therefrom so slowly that practically the whole quantity of liquid remains in the bath. The substrates are brought directly from the liquid into contact with a vapor not condensing thereon of a substance which is miscible with the liquid and yields, when mixed therewith, a mixture having a surface tension lower than that of the liquid. The wafers are pulled from the bath so slowly that practically the whole quantity of liquid remains in the bath (i.e. 5 cm/min). However, the reference discloses that it may be desirable to keep a thin film of water on the surface of the wafer after it is pulled from the bath. The solvents used are extremely volatile with a solubility in water which is higher than 1 g/L. Octane is not compatible with the process. The process disclosed requires removal of the wafer from the cassette prior to pulling the wafer out of the bath and then reinserting the wafer in the cassette prior to vapor drying. The vapor does not condense on the surface of the wafer.
U.S. Pat. No. 4,984,597 discloses a similar drying process wherein a drying vapor is supplied to the surfaces in such a manner that the vapor replaces the rinsing fluid by directly displacing the rinsing fluid on the surfaces at such a rate that substantially no liquid droplets are left on the surfaces after replacement of the rinsing fluid with drying vapor. Preferably, the drying vapor is provided from above the objects in a fully enclosed, hydraulically full system and the drying vapor pushes the rinsing liquid off the surfaces as the liquid level recedes downwardly.
There are, however, several disadvantages associated with the above-mentioned chemical vapor drying processes. First, vaporization of organic solvents is a fire hazard because vaporization is usually achieved by heating. Second, the large volume of vapor needed for the process creates emission and reclamation problems. Third, all of the above-mentioned techniques use additional hardware which adds expense and requires more cleanroom floorspace. Third, chemical vapor drying of wafers is fairly time consuming. Fourth, drying time for standard process wafer handling cassettes, boats or end effectors is slow and drying is not always complete. Fifth, vapor drying allows for only a limited selection of solvents (i.e. those that form an azeotrope with the rinsing fluid such as isopropanol and water) for acceptable drying results.
It would also be desirable to conduct all processing steps, including drying in a single tank.
Accordingly, it is an object of this invention to provide a process for drying surfaces of objects such as semiconductor wafers in the same tank in which the other processing steps are completed.
It is another object of this invention to provide a process for drying surfaces of objects such as semiconductor wafers wherein streaking or spotting on the wafer surface is minimized thus providing for a higher yield.
It is further an object of this invention to provide a drying process for the surfaces of objects such as semiconductor wafers wherein minimal mechanical stress is placed on the wafer, there is minimal contamination of the wafer and the wafer does not receive a static charge during drying.
It is yet a further object of this invention to provide a drying process for the surfaces of objects such as semiconductor wafers that does not require the generation, handling and reclamation of large amounts of organic vapor.
It is still further an object of this invention to provide a drying process for the surfaces of objects such as semiconductor wafers with reduced process time and reduced wafer handling.
These and other objects of the present invention will become apparent upon a review of the following specification and the claims appended thereto.